8 Papers from the Department of Marine Biology. 



recorded by Hafele,they are clustered thickly on the proximal part of 

 the limb, while the last two or three joints are entirely free. Occasion- 

 ally external sacs are found on the chelipeds and even the third pair of 

 maxillipeds, though I have never seen them, as Hafele notes, on the 

 eye-stalks of the host. Posteriorly they extend to the uropods and to 

 both surfaces of the telson when the infection is heavy. Rarely, too, 

 they spring from the pleura of the abdominal segments, but never from 

 the terga and sterna. 



On the last day of work at Murray Island (October 25) I obtained a 

 swimming crab, Thalamita prymna, which was parasitised by another 

 species of Thom,psonia (pi. 1, fig. 3). The thoracic legs, including both 

 the chelae (text-figure 1), were thickly beset with elongated lemon-yellow 



FIG. 1. 



Chela of Thalamita prymna para- 

 sitised by Thompsonia. X2. The 

 number and thick distribution of 

 the external sacs along the margins 

 of the appendage are shown about 

 as in life. 



sacs containing Cypris larvse, much larger than those on Synalpheus. 

 The sacs were nearly 3 mm. in length, counting the peduncle. It 

 was impossible, as we were on the eve of departure, to obtain further 

 material of this form, but the capture gave me an opportunity of 

 examining the genus on its typical host, a Brachyuran. Since the swim- 

 ming crab is much larger than the Alpheid, the number of external sacs 

 is much greater on the former; a single ambulatory limb may have 

 nearly a hundred external sacs upon it. Altogether there were cer- 

 tainly more than 500 on my specimen of Thalamita, while the number 

 on Synalpheus never reaches 200. 



Dr. W. T. Caiman, of the Natural History Department of the 

 British Museum, has been kind enough to entrust to me a specimen of 

 Actcea ruppellii (pi. 1, fig. 4) from South Africa, which also bears a large 

 number of sacs of a species of Thompsonia. This species probably 

 differs from that found on Thalamita, the sacs being much smaller and 

 pear-shaped. 



As far as I am able to judge, there is no essential difference between 

 the forms of Thompsonia which infest Macrurous and Brachyurous 



